The left circumflex artery (LCx) arises from the left main coronary artery and courses in the left atrioventricular groove. It supplies the lateral and posterolateral left ventricular wall and, in left-dominant systems, the inferior wall and septum.
Origin and Course
The LCx originates at the bifurcation of the left main coronary artery, branching at approximately 90 degrees to the LAD. It passes posteriorly and leftward in the left atrioventricular groove.
Course segments:
- Proximal LCx: From origin to the first obtuse marginal branch
- Mid LCx: From first to second obtuse marginal branch
- Distal LCx: Beyond the second obtuse marginal branch; terminates in the posterior atrioventricular groove
The LCx terminates variably depending on coronary dominance (see section on dominance).
Branches
Obtuse Marginal Branches (OM Branches)
The obtuse marginal branches are the principal branches of the LCx, supplying the lateral and posterolateral left ventricular wall.
Characteristics:
- 1-4 OM branches
- Course over the obtuse margin of the heart (the rounded border between the anterior and posterior surfaces)
- OM1 (first obtuse marginal) is usually the largest
- Run parallel to each other toward the apex
Other Branches
Left atrial circumflex branch: Supplies the left atrium
Left marginal artery: Large branch that courses toward the apex along the left margin
Posterolateral branches (in left-dominant systems): Continue beyond the OM branches to supply the posterior left ventricle
Posterior descending artery (in left-dominant systems): Descends in the posterior interventricular groove
Sinoatrial nodal artery (variant): Arises from the LCx in approximately 10-15% of hearts
Atrioventricular nodal artery (in left-dominant systems): Supplies the AV node when the LCx gives off the PDA
Dimensions
| Segment | Typical Diameter |
|---|---|
| Proximal LCx | 3.0-4.5 mm |
| Mid LCx | 2.0-3.5 mm |
| Distal LCx | 1.5-2.5 mm |
| Obtuse marginal branches | 2.0-3.5 mm |
Myocardial Territory Supplied
| Territory | Supply |
|---|---|
| Lateral left ventricular wall | OM branches |
| Posterolateral left ventricle | Distal LCx / posterolateral branches |
| Left atrium | Left atrial circumflex branch |
| Posterior papillary muscle (mitral) | LCx |
| Posteromedial papillary muscle | LCx (in right-dominant) or PDA (in left-dominant) |
| Sinoatrial node | LCx (10-15%) |
| AV node | LCx (if left-dominant, 10-15%) |
| Posterior septum | LCx (if left-dominant via PDA, 10-15%) |
Anatomic Variants
LCx Dominance and Termination
The LCx termination depends on coronary dominance:
| Dominance | LCx Termination | Frequency |
|---|---|---|
| Right-dominant | Terminates as OM branches; does not reach the crux | 70-80% |
| Co-dominant | Gives off posterolateral branches; small or absent PDA | 10-15% |
| Left-dominant | Continues to the crux and gives off PDA and posterolateral branches | 10-15% |
Abrupt LCx Origin
In some individuals, the LCx arises at an acute angle from the LMCA, creating a sharp bend that can complicate catheter engagement during angiography.
LCx Arising from the Right Sinus
Rare variant where the LCx originates from the right coronary sinus, coursing posterior to the aorta to reach the left atrioventricular groove.
Clinical Significance
LCx Myocardial Infarction
Characteristics of LCx-related MI:
- Often presents as NSTEMI (non-ST elevation MI) rather than STEMI
- Lateral wall involvement: ST elevation in I, aVL, V5, V6
- Posterior wall involvement: ST depression in V1-V3 (reciprocal changes)
- May be electrically silent (no diagnostic ECG changes)
- Posterior infarction may require right-sided leads
Types of LCx Occlusion:
| Occlusion Level | Territory Affected | ECG Findings |
|---|---|---|
| Proximal LCx | Lateral wall, posterolateral, posterior papillary muscle | ST elevation I, aVL, V5, V6 |
| Dominant LCx (proximal) | Lateral, posterior, inferior, septal | Large territory, may simulate RCA infarction |
| OM branch | Limited lateral wall | Variable, may be normal |
| Distal LCx | Posterolateral | Often normal ECG |
Mitral Regurgitation in LCx Disease
The posteromedial papillary muscle receives blood supply from the LCx (and the PDA depending on dominance). LCx infarction can cause:
- Papillary muscle dysfunction
- Acute mitral regurgitation
- Pulmonary edema
LCx in Percutaneous Coronary Intervention
Challenges:
- Tortuous course may require extra support catheters
- Acute angle of origin may make engagement difficult
- Deep seating of guide catheter may cause ostial dissection
- OM branch engagement may require specialized wires
LCx Vasospasm (Prinzmetal Angina)
The LCx is commonly involved in coronary vasospasm, presenting as:
- Angina at rest
- ST elevation during pain (transmural ischemia)
- Normal coronary arteries on angiography (with provocative testing)
- Good response to calcium channel blockers
LCx in Mitral Valve Surgery
The LCx courses in the left atrioventricular groove, directly adjacent to the mitral annulus. During mitral valve surgery:
- Sutures in the posterior mitral annulus risk LCx injury
- LCx occlusion can cause lateral wall infarction
- Anatomic awareness is critical for surgical safety